1. Field of the Invention
The present invention relates generally to quadrature modulators, and more particularly to suppressing carrier leakage in quadrature modulators.
2. Related Art
FIG. 1 is a block diagram of a conventional quadrature modulator 102 comprising two bi-phase modulators 104, 106, a phase shifter 108, and a combiner 110. The quadrature modulator 102 operates generally as follows.
The modulator 104 modulates a first carrier signal 116 using an in-phase (I) baseband signal 112 to thereby produce a modulated in-phase signal 120. The phase shifter 108 receives the first carrier signal 116 and generates therefrom a second carrier signal 124, wherein the second carrier signal 124 is in quadrature with the first carrier signal 116 (that is, the second carrier signal 124 is 90 degrees out of phase with the first carrier signal 116).
The modulator 106 modulates the second carrier signal 124 with a quadrature (Q) signal 114 to thereby produce a modulated quadrature signal 122, wherein the modulated quadrature signal 122 is in quadrature with the modulated in-phase signal 120. The modulated in-phase signal 120 and the modulated quadrature signal 122 are then combined in-phase by the combiner 110 to thereby produce a radio frequency (RF) output signal 118.
The in-phase signal 112 and the quadrature signal 114 typically range in frequency from approximately 0 Hz to 2 MHz. The frequency of the first carrier signal 116 is typically approximately 900 MHz. These frequencies are provided for illustrative purposes only, and may be other values. As will be appreciated, the transmit frequency of the RF output signal 118 is substantially equal to the frequency of the first carrier signal 116.
Imperfections in the modulators 104, 106 result in the generation of undesired mixing products, such as signals having frequencies equal to some or all of the frequencies of the signals 112, 114, 116, 124 which were originally mixed by the modulators 104, 106. These undesired mixing products are combined with the modulated in-phase signal 120 and the modulated quadrature signal 122 by the combiner 110 to produce the RF output signal 118. In other words, the RF output signal 118 includes undesired signal components having frequencies equal to the frequency of the in-phase signal 112, the frequency of the quadrature signal 114, the frequencies of the first and/or second carrier signals 116, 124, and/or combinations of these frequencies.
Such undesired signal components having frequencies equal to the frequencies of the in-phase signal 112 and the quadrature signal 114 are easily filtered and eliminated from the RF output signal 118 since their frequencies (approximately 0 Hz to 2 MHz) are so different from the transmit frequency (approximately 900 MHz) of the RF output signal 118.
In contrast, the signal components having frequencies equal to the frequencies of the first and second carrier signals 116, 124 are difficult to eliminate from the RF output signal 118 since their frequencies are so close to the transmit frequency of the RF output signal 118. Since they originate from the first and second carrier signals 116, 124, these undesired signal components are called carrier leakage signals, or simply carrier leakage.
With regard to quadrature modulators, a DC offset applied to the quadrature baseband channel or rail (that is, added to the quadrature baseband signal) operates to cancel the quadrature component of the carrier leakage. Similarly, a DC offset applied to the in-phase baseband channel operates to cancel the in-phase component of the carrier leakage. Thus, it is possible to substantially eliminate carrier leakage by applying appropriate DC offsets to the quadrature and/or in-phase baseband channels.
Also, carrier leakage can be substantially eliminated by appropriately selecting component values to thereby balance the bi-phase modulators contained in quadrature modulators.
However, the application of DC offsets to the quadrature and/or in-phase baseband channels and the selection of component values to balance the bi-phase modulators do not represent complete solutions since carrier leakage varies with many factors, such as temperature, frequency, load impedance, and carrier power. Consequently, the component values selected to balance the bi-phase modulators and to thereby eliminate carrier leakage when the operating temperature is M degrees will probably not be adequate to eliminate carrier leakage when the operating temperature changes to N degrees.
Similarly, the DC offsets calculated to eliminate carrier leakage when the carrier frequency is X MHz will probably not be adequate to eliminate carrier leakage when the carrier frequency changes to Y MHz. This is particularly true for direct modulation (where a baseband signal modulates a carrier signal whose carrier frequency is equal to the transmit frequency) where the carrier frequency may often change.
Thus, what is required is a quadrature modulator wherein carrier leakage is suppressed even when the carrier leakage changes due to factors such as temperature, frequency, load impedance, and carrier power.